You could soon rappel down Willis Tower's Skydeck

Want to rappel down the side of Willis Tower? Or take an outdoor stroll on a balcony 102 stories up?

You may soon have your chance.

Blackstone Group, the owner of the iconic Chicago skyscraper, is drawing up plans for a $20 million expansion of the Skydeck, the popular observatory on the building's 103rd floor. One attraction in the works would allow visitors to rappel—or descend via rope—from the 103rd to 102nd floor inside a two-story enclosed glass box on the side of the tower, according to a recent report from Morningstar Credit Ratings.

If that doesn't get your heart pumping, you could try the Ledgewalk, an outdoor glass ledge where visitors can stretch their legs, breathe in some fresh, rarefied air and take in the views—clipped into a safety harness, of course, the report said.

Blackstone is mulling those ideas as part of a larger $546 million renovation of the 110-story skyscraper at 233 S. Wacker Drive, the tallest in the Western Hemisphere. The New York-based real estate firm aims to boost the property's income—and value—by attracting new tenants to its revamped office space and transforming its lower floors into a retail destination and food hall. Blackstone also sees a big opportunity with the Skydeck, already one of the city's most popular tourist destinations, drawing 1.7 million visitors a year.

"The Willis Tower has been under-managed and under-invested in for a number of years, and that changes today," Jon Gray, Blackstone's global head of real estate, said when the firm unveiled its rehab plan in early February.

Blackstone shared more details of its project with Morningstar because the landlord is refinancing the property with a two-year, $1 billion loan that's being converted into commercial mortgage-backed securities and sold to bond investors. Morningstar Credit Ratings, the debt-rating unit of Chicago-based Morningstar, provides an in-depth analysis of the building and Blackstone's plan in its report, which reveals specifics that Blackstone didn't disclose last month.

Paula Chirhart, a Blackstone spokeswoman, declined to comment, saying that any potential changes to the Skydeck are in "a preliminary stage."

The Skydeck's income has jumped over the past few years due largely to the popularity of the Ledge. That attraction, which opened in 2009, allows visitors to step into glass boxes that extend out from the building and look down to Wacker Drive 103 stories below.

Operating income at the Skydeck rose to $29.9 million in 2016, up 22 percent from 2014, according to the Morningstar report. The observatory also is an extremely lucrative business, generating an operating profit margin of 84 percent last year.

But Blackstone believes it still has untapped potential. The Skydeck can only accommodate about 600 guests at one time, and the wait to get in can take up to two hours in the summer, according to Morningstar. So Blackstone, which paid $1.3 billion for Willis Tower in 2015, wants to expand the Skydeck to the entire 102nd floor, effectively doubling its capacity, the report said.

In addition to the rappelling and Ledgewalk proposals under consideration, Blackstone plans to expand retail and food/drink offerings at the Skydeck, "significantly enhancing the duration of the experience and keeping visitors on site and spending money," Morningstar said. A rendering included in the report also shows an exterior glass-enclosed staircase between the two floors, another potential adrenaline-inducing addition.

After the $20 million expansion, Blackstone expects Skydeck annual attendance to jump to 2.1 million and its revenue to rise 39 percent to $51 million, according to the Morningstar report. Morningstar's revenue estimate is more conservative, at $40 million.

Social media reaction to the new Skydeck ideas ranged from excitement to something approaching vertigo: